The purpose of this contract is to establish rodent foundation and production colonies and produce disease-free, genetically homogeneous and microbiologically defined Fischer 344 rats and B6C3F1 hybrid mice and small colonies of special strains such as transgenic animals for the NTP and NIEHS toxicology research and testing programs. Genetically defined inbred pedigreed animals associated with defined microflora are being produced in the rodent foundation colonies to maintain pedigreed expansion colonies. Offspring from these expansion colonies are being used to supply breeders to produce contractually specified number of each species. Rodent production is being modulated to supply adequate number of animals for the NTP and NIEHS studies without excess production. The Environmental Toxicology Program (ETP) under the National Toxicology Program (NTP) organizes and conducts a comprehensive interagency testing and research program focused on determining potential human health hazards due to exposure to chemicals. The toxicologic evaluation of chemicals is generally conducted through a sequence of experiments that involve acute, subacute, subchronic and chronic exposure of laboratory animals to chemical substances. Rats, mice and other small laboratory animals are appropriate species for evaluating toxicologic and carcinogenic properties of chemicals. Fischer 344 rats and B6C3F1 hybrid mice are the selected experimental animal species for evaluation of chemicals for toxicologic and carcinogenic properties under NTP. Centralized production facilities for Fischer 344 rats and B6C3F1 hybrid mice with ho mogeneous genetic properties and defined microbial status will insure an adequate and continuous supply of defined quality animals for the toxicology research and testing programs. Under this contract foundation and production colonies of transgenic mouse strains such as TG.AC (skin cancer model), p53 deficient (knockout), TG.NK (breast cancer model), and APC deficient/knockout (colon cancer model) are also established and producing mice for NTP and NIEHS Investigators and their collaborators.